Mean Girls in Elementary School - Anyone have a speaker to recommend?

I'm in Oakland Unified School District - California. This year we have had 5th grade girls in particular, who where down right mean to other students, one could even call it bullying. At first I thought, it's so near the end of the year, these girls, and the problem would be gone because nearly all of them do not have younger siblings in the school.

However, I see a group of 3rd grade girls acting similarly - they want to exclude some girls, so if they see someone talking to the girl they want to exclude, they will walk up and take away the girl speaking and go off to talk in a group, turn back, point, laugh, toss their heads back and walk away.

I am ashamed to say that when a former student, whose mom was a psycho-therapist, made an L for loser sign with her fingers I comforted the child who was targeted and did not confront the girl and her mother.

My question is - does anyone have a list of speakers who will come to a school and talk about the issue? Is there a way to get this in the open so that it is looked for in daily interactions?

I have watched this mean girl dynamic two times in two days. Enough is enough.